Iodine deficiency disorders are major problems in most developing countries. According to WHO more than one billion people worldwide are at risk for iodine deficiency. However, prophylaxis programs based on the distribution of iodized salt have been unsuccessful in developing countries for socioeconomic reasons. We tested the efficacy of iodination of water with sodium iodide incorporated into silicone matrices (Rhône-Poulenc-Rorer-Doma). Nine such matrices were placed into each of the 198 wells supplying more than 90% of the 85,037 inhabitants of the Nana-Grebizi prefecture (figure 1). Efficacy was evaluated by following classical markers of iodine deficiency in representative samples of the population (figure 2): goiter size was measured immediately before (T = 0, n = 3,090) and twelve months after (T = 12, n = 2,645) installation of the matrices; and urinary iodine concentrations were assayed at T = 0 (n = 319), 6 (n = 304) and 12 (n = 261). The prevalence of goiter was 60.9% (visible goiter 10.7%, cretinism 0.7%) and the median urinary iodine concentration was 2.1 (95% confidence interval 2.0-2.3) micrograms/dl. Thus the population suffered from severe iodine deficiency. All villages were severely affected, despite inter-village variations (tables 1 and 2, figure 4). Twelve months after iodination of wells, the overall prevalence of goiter fell to 44.5% (p < 0.0001, table 1 and figure 3) and that of visible goiter to 2.5% (p < 0.0001). With the exception of one village (Bokanzi) the prevalence of goiter decreased by 18.6 to 56.3% (table 1).(ABSTRACT TRUNCATED AT 250 WORDS)